“Michelle” writes to us with a question about her dog:
“My 9 year old Shih Tzu is prone to sebaceous cysts. He had one on his shoulder for a few months. It burst and I cleaned it up with Geri stat soap. I squeezed out some whitish fluid but it was not as easily cleaned out as others that he has had were. The following week it was a bit puffy and when cleaned, more whitish fluid came out. It stayed small for a few weeks then got a little bigger then disappeared. My veterinarian said that it was just a cyst but I am worried about a mast cell tumor.“
Mast cells are a part of the immune system, protecting our pets from invaders that could make them ill. Like any cell that becomes autonomous and begins multiplying and dividing without the ability to turn off, mast cells can become cancerous. They are among the growths in the category of round cell tumors, and are sometimes called mastocytoma.
Of the many types of cancers you don’t want your pets to have, mast cell tumors are among the worst.
In both dogs and cats mast cell tumors most commonly appear as skin growths. They can also be in primary intestinal tumors as well as in the circulating blood as a type of leukemia.
Here is a good example of how tricky mast cell tumors can be. I had a patient a couple of years ago who came in with a lump on the left rear leg. There was nothing “showing” to the outside, the lesion was completely haired and not inflamed. The only thing that concerned me was the position of the mass and the fact that it appeared to infiltrate the surrounding tissues. Based on that concern, I referred the patient to a nearby specialty center for board-certified veterinary surgeons to remove it. In their protocol, however, masses automatically go through the Oncology service first. The oncologist’s take on the lump was very different from mine.
“As soon as I walked in the room I knew it was a mast cell tumor,” she told me on the phone after her examination. We obtained a fine needle aspirate of the mass, and we can stage it from the results. I discussed with the owner the options of surgery, chemotherapy and radiation. Appropriate treatment will depend on the outcome of the staging.”
When we finished our conversation I hung up the phone shaking my head. Schnockered by a mast cell tumor again!
Even though we are taught in veterinary medical school not to diagnose masses by looking at them, there are many growths that we can make an educated guess about prior to receiving a pathology report. Obviously a board-certified veterinary oncologist has many more opportunities to evaluate growths than a general practitioner, thus she was able to use her experience to correctly evaluate this little dog’s mass. Of course, she didn’t stop there. The oncology doctor obtained specimens for herself and for the pathologist’s evaluation to confirm the diagnosis and to prepare for proper treatment.
Treatment. Mast cell tumors create yet another quagmire.
Most treatment protocols create four categories for mast cell tumors and some create subcategories as well. Aggressiveness of treatment is determined by staging. Those mast cell tumors staged as the mildest may be fully treated with surgery alone if wide margins are obtained. “Margins” refers to the amount of normal tissue between the abnormal growth and outer edge of the surgery site, as observed by the pathologist under his microscope.
Patients with mast cell tumors staged as the most aggressive may be candidates for initial surgical removal followed by radiation and/or chemotherapy. It is not uncommon for cancerous mast cells to travel from a mass to nearby lymph nodes or even lymph nodes throughout the body.
Treatment and prognosis both depend on the outcome of staging. Prognosis may change according to response to treatment. Every case is taken as an individual.
Our advice for Michelle is to make an appointment for her pet to have some sort of testing to reassure her with a specific diagnosis. That might be obtained by fine needle aspirate, cytology or biopsy.